This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-048042, filed Feb. 25, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a color reproduction system including a color image processing apparatus for obtaining an input color image signal used to display any desired color on a color image display.
In general, display apparatuses such as CRT displays, liquid crystal displays, liquid crystal projectors, etc. are widely used as means for reproducing a digital color image input by an image input device such as a color scanner, a digital camera, etc.
In these display apparatuses, various colors are usually reproduced by additive color mixture of three primary colors RGB (red, green and blue).
The color gamut (color reproduction area) that can be reproduced on a display is limited to a region in which each color is expressed as the sum of color vectors of the three primary colors in the three dimensional color space. For example, in a CRT display for reproducing colors using three kinds of fluorescent substances, i.e. Red, Green and Blue, the color reproduction area corresponds to a hexahedron having corners (0, 0, 0), (Xr, Yr, Zr), (Xg, Yg, Zg), (Xb, Yb, Zb), (Xr+Xg, Yr+Yg, Zr+Zg), (Xg+Xb, Yg+Yb, Zg+Zb), (Xb+Xr, Yb+Yr, Zb+Zr), (Xr+Xg+Xb, Yr+Yg+Yb, Zr+Zg+Zb), assuming that (Xr, Yr, Zr), (Xg, Yg, Zg) and (Xb, Yb, Zb) represent X, Y and Z data which belongs to the CIE1931 color system (XYZ color system) which are obtained when RGB fluorescent substances emit maximum light respectively.
FIG. 16 schematically shows a color reproduction area in the XYZ space of such a three-primary-color display. If this area is expressed using an xy chromaticity diagram, it corresponds to the interior of a triangle formed by the chromaticity of each primary color, as is shown in FIG. 17.
In the display using the additive color mixture of three primary colors RGB as the color reproduction principle, the relationship between RGB signal values input to the display and XYZ values of each color displayed thereon is uniquely determined in the color reproduction area of the display.
If it is assumed that the emission spectrum of each primary color is independent of the intensity of any other primary color, and the relative spectral distribution does not depend upon the intensity of emission (i.e., the chromaticity does not vary even if the emission intensity varies), the X, Y and Z data of a to-be-displayed color corresponding to input RGB values are given by the following formula:                                           (                                                            X                                                                              Y                                                                              Z                                                      )                    =                                    (                                                                    Xr                                                        Xg                                                        Xb                                                                                        Yr                                                        Yg                                                        Yb                                                                                        Zr                                                        Zg                                                        Zb                                                              )                        ⁢                          xe2x80x83                        ⁢                          (                                                                                          R                      xe2x80x2                                                                                                                                  G                      xe2x80x2                                                                                                                                  B                      xe2x80x2                                                                                  )                                      ⁢                  
                ⁢                              R            xe2x80x2                    =                                    γ              r                        ⁡                          (              R              )                                      ⁢                  
                ⁢                              G            xe2x80x2                    =                                    γ              g                        ⁡                          (              G              )                                      ⁢                  
                ⁢                              B            xe2x80x2                    =                                    γ              b                        ⁡                          (              B              )                                                          (        1        )            
where Xr, Xg and Xb represent the respective X data items obtained when the RGB fluorescent substances emit maximum light, similarly, Yr, Yg and Yb represent the respective Y data items obtained when the RGB fluorescent substances emit maximum light, and Zr, Zg and Zb represent the respective Z data items obtained when the RGB fluorescent substances emit maximum light. Further, xcex3r, xcex3g and xcex3b represent functions indicating the relationship between the input signal value and the output luminescence, and Rxe2x80x2, Gxe2x80x2 and Bxe2x80x2 represent signal values normalized so that they will be xe2x80x9c1xe2x80x9d when the RGB fluorescent substances emit maximum light.
From the reverse relationship of the above, input RGB values for displaying desired X, Y and Z data are given by the following formula:                               R          =                                    γ              r                              -                1                                      ⁡                          (                              R                xe2x80x2                            )                                      ⁢                  
                ⁢                  G          =                                    γ              g                              -                1                                      ⁡                          (                              G                xe2x80x2                            )                                      ⁢                  
                ⁢                  B          =                                    γ              b                              -                1                                      ⁡                          (                              B                xe2x80x2                            )                                      ⁢                  
                ⁢                              (                                                                                R                    xe2x80x2                                                                                                                    G                    xe2x80x2                                                                                                                    B                    xe2x80x2                                                                        )                    =                                                    (                                                                            Xr                                                              Xg                                                              Xb                                                                                                  Yr                                                              Yg                                                              Yb                                                                                                  Zr                                                              Zg                                                              Zb                                                                      )                                            -                1                                      ⁢                          (                                                                    X                                                                                        Y                                                                                        Z                                                              )                                                          (        2        )            
where xe2x80x9cxe2x88x921xe2x80x9d indicates inverse function and inverse matrix. Thus, in the three primary color display, it is easy to model the relationship between XYZ data and RGB values, and a conversion method using matrix conversion and gradation correction is generally used as described in xe2x80x9cCOLOR IMAGE DUPLICATIONxe2x80x9d (written by Johji Tajima and published by Maruzen Co., Ltd.).
If in the three primary color display, a point (X, Y, Z) falls out of the color reproduction area of the display, one of Rxe2x80x2, Gxe2x80x2 and Bxe2x80x2 obtained from the formula (2) is xe2x80x9cnegativexe2x80x9d or xe2x80x9chigher than 1xe2x80x9d.
As described above, in the display apparatus using, as the principle, the additive color mixture of three primary colors, the area determined by the chromaticity coordinates of each primary color is the color reproduction area. In order to enlarge the color reproduction area, it is considered to increase the chroma of each primary color or to increase the number of primary colors.
For example, as is disclosed in NHK Technology Research Published Documents (published by NHK Broadcasting Technology Research, Tokyo 1995), an attempt has been made to realize a wider color reproduction area than the conventional three-primary-color display by using four primary colors instead of three primary colors.
FIGS. 18 and 19 show a color reproduction area in the XYZ space of the four primary color display, and a color reproduction area in the xy chromaticity diagram, respectively.
Also in the case of a multi-primary-color display using a number N of primary colors not less than four primary colors, the XYZ data of each to-be-displayed color corresponding to signal values can be given by the following formula that is obtained by generalizing the formula (1):                                                         (                                                                    X                                                                                        Y                                                                                        Z                                                              )                        =                                          (                                                                            Xc1                                                              Xc2                                                              Xc3                                                              ⋯                                                              XcN                                                                                                  Yc1                                                              Yc2                                                              Yc3                                                              ⋯                                                              YcN                                                                                                  Zc1                                                              Zc2                                                              Zc3                                                              ⋯                                                              ZcN                                                                      )                            ⁢                              xe2x80x83                            ⁢                              (                                                                                                    C1                        xe2x80x2                                                                                                                                                C2                        xe2x80x2                                                                                                                                                C3                        xe2x80x2                                                                                                                                                xe2x80x83                                                                                                                                                CN                        xe2x80x2                                                                                            )                                              ⁢                      
                    ⁢                                    C1              xe2x80x2                        =                          γ              ⁢                              xe2x80x83                            ⁢              1              ⁢                              (                C1                )                                              ⁢                      
                    ⁢                                    C2              xe2x80x2                        =                          γ              ⁢                              xe2x80x83                            ⁢              2              ⁢                              (                C2                )                                              ⁢                      
                    ⁢                      C3            xe2x80x2                    =                      γ            ⁢                          xe2x80x83                        ⁢            3            ⁢                          (              C3              )                                      ⁢                  
                ⁢                  
                ⁢                              CN            xe2x80x2                    =                      γ            ⁢                          xe2x80x83                        ⁢                          N              ⁡                              (                CN                )                                                                        (        3        )            
The conversion from XYZ data into signal values, which is considered a reverse relationship with respect to the formula (3), is not directly executed except for the surface of the color reproduction area of the multi-primary-color display.
It is necessary to execute conversion with signal values corresponding to X, Y and Z determined on the basis of certain conditions. For example, Japanese Patent Application KOKAI Publication No. 6-261332 discloses a color conversion method employed in a multi-primary-color display.
In a conversion method as a first invention of the application, color reproduction is executed using the linear sum of three primary colors selected on the basis of the chroma values of input colors. In this method, accurate color reproduction can be executed within a range in which reproduction can be realized on the basis of the selected three primary colors. However, since the method does not consider a color reproduction area in the direction of brightness in the multi-color display, it cannot deal with all reproducible input colors.
Moreover, in a second invention disclosed in the above publication, linear conversion is executed using multiple primary colors. In this case, however, it is not guaranteed whether a solution that imparts a positive value to any primary color signal can be obtained. Accordingly, there is a case where even a reproducible input color cannot accurately be reproduced.
To execute accurate color reproduction, the color conversion method for display apparatuses including a multi-primary-color display that uses four or more primary colors needs to satisfy the following conditions:
First, colorimetrically accurate color reproduction can be executed.
Second, there is continuity between XYZ data and signal values.
Third, conversion that satisfies the above conditions is executed within all the color reproduction area.
In addition to the above conditions, high-speed conversion is demanded. Moreover, it is desirable that colorimetrically accurate conversion can be executed even where additive color mixture cannot strictly be executed (i.e. even where the emission spectra of primary colors are not independent of each other).
It is the object of the invention to provide a color reproduction system including a multi-primary-color display which can realize colorimetrically accurate color reproduction, can impart continuity between XYZ data and signal values, and can use its color reproduction area to the maximum extent.
According to an aspect of the invention, there is provide a color reproduction system having color image display means, and color conversion means for converting input tristimulus values into a color image signal to be input to the color image display means, wherein the color conversion means comprising: maximum brightness signal calculating means for calculating three color reproduction area corner vectors that indicate three corners of a plane constituting a color reproduction area surface of the color image display means, the three color reproduction area corner vectors surrounding vectors of the input tristimulus values, the maximum brightness signal calculating means also calculating a weighting factor obtained when the input tristimulus values are expressed as a linear sum of tristimulus values of the color reproduction area corner vectors; and color image signal calculating means for calculating a color image signal used to display the input tristimulus values, on the basis of signal values corresponding to the color reproduction area corner vectors and the weighting factor.
According to another aspect of the invention, there is provided a color reproduction system having color image display means, and color conversion means for converting input tristimulus values into a color image signal to be input to the color image display means, wherein the color conversion means comprising: chromaticity area calculating means for calculating a chromaticity area surrounded by corners of planes constituting a color reproduction area surface of the color image display means; matrix calculating means for calculating matrix data that provides a relationship between tristimulus values of the corners and the color image signal; matrix data storage means for storing the matrix data in relation to the chromaticity area; and color image signal calculating means for converting the input tristimulus values into the color image signal, using part of the matrix data selected on the basis of chromaticity values of the input tristimulus values.
The color reproduction system constructed as above can convert image data XYZ into signals to be input to a display that realizes colorimetrically accurate reproduction, and can satisfy the conditions that there must be continuity between XYZ data and signal values, and that the color reproduction area must be used to the maximum extent. As a result, the system can realize accurate correspondence between the XYZ data and the signal values even in the case of a display in which additive color mixture cannot strictly be executed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.